The invention relates to the heating, melting, undercooling and solidification of high-melting temperature metals (1500 to 3000 degrees kelvin) and metal alloys in a containerless low gravity environment during space flight.
The device is particularly useful for preparing the various classes of metals, alloys and glasses which include metastable alloys and compounds; highly reactive metals, amorphous metals as well as conventional glasses and spherical single crystals.
Heretofore, apparatus and devices have been provided for low gravity containerless processes which have been classified into two groups, ground base and space flight type processes. Ground based processes for high temperature metals and alloys in a low gravity container less environment have been limited mainly to drop tubes. A drop tube is a long vertical tube in which molten samples are allowed to free fall through a cooling gas to enhance cooling rates. Undercoolings can not be directly measured but must instead must be estimated using complex cooling models. Drop tubes are for only short free fall and processing times on the order of less than five seconds.
Previous and current methods for containerlessly processing samples during a space flight may be grouped into two categories, electromagnetic levitators and acoustic levitators. Electromagnetic levitators operate by passing a high frequency oscillating current through a previously formed coil. An electromagnetic field is then set up inside the coil which can levitate a heated sample. In low gravity of space, the electromagnetic levitator is used to heat and position a sample. The sample is cooled by controlling the coil current or by passing a cooling gas through the coil and around the sample. To facilitate heating, an optional electron beam may be used with an electromagnetic levitator.
U.S. Pat. Nos. 3,476,170 and 3,363,081 describe electromagnetic levitators for use in round base systems which employ electromagnetic fields for levitation and melting. However, these devices induce varied stirrring in the sample and may therefore limit the degree of undercooling expected.
Acoustic levitators position the sample by use of resonant sound waves. To heat and melt a sample auxiliary heating must be used. Currently, all auxiliary heating is by laser, radiant energy or electron beam.
U.S. Pat. Nos. 3,365,184 and 3,270,118 are typical of the type apparatus which employ electron beam furnaces for the melting of material specimens which are not containerless or low gravity.
Another major disadvantage of the prior art devices is that operating temperatures have been limited below 1500 degrees Kelvin. Unfortunately, the more interesting alloy systems must be processed at temperatures above 1500 degrees Kelvin. These metal alloy systems include super alloys, refractory metals and alloys, most metallic glass systems, and pure metals with melting temperatures above 1500 degrees Kelvin.
While electromagnetic levitators have been used in systems where the temperatures have been above 1500 degrees Kelvin, these applications have been limited to ground base systems and not low gravity. These systems may also have the overriding problem that levitation by external forces can cause surface disturbances in the specimen resulting in dynamic nucleation or mechanical disturbance in the surface. This adversely affects the ability to undercool the specimen.
The advantages of undercooling are the following. In order to form non-equilibrium or metastable states within alloy systems, this can be done by simply increasing the solid solubility of the second constituent in an alloy or actually forming non-equlibrium crystal structures which can not be formed by slow cooling. This would offer the advantages of enhanced mechanical, electrical or superconducting properties.
Accordingly, an object of the present invention is to provide apparatus affording a quiescent containerless low gravity environment in space for undercooling and solidifying materials.
Another object of the present invention is to provide a quiescent containerless low gravity environment for heating, melting, undercooling and solidifying high temperature metals and metal alloys at temperatures above 1500 degrees Kelvin.
Still another object of the invention is to provide a quiescent containerless low gravity environment for processing high temperture metals and metal alloys in a low gravity space environment wherein the material specimens may be levitated without external forces which can cause dynamic nucleation and affect undercooling.